I wouldn't be so quick to jump to the conclusion that Microsoft's Natal will be gimmicky any more than I'll accept the argument than Nintendo's Wii is gimmicky. The problem isn't the systems, it's the developers, and it extends far beyond motion sensors.
When is the last time a developer came up with something new that wasn't gimmicky? Leave aside everything we've had for decades: RPGs, FPSs, RTSs. The only thing that developers have added to these are little mechanics, almost all of which are gimmicks. In fact, I'm having trouble thinking of any real developements. Emotes, customizable avatars, voice control; all gimmicks that we've come to utilize, but have added little to the games themselves. Recently some games have tried new things, like the inventory system and combining items in Alone in the Dark, or integrating the player's status into their character in Dead Space, but even these small changes did nothing substantial.
Perhaps the only recent advances that haven't been pure gimmicks are the things like DDR dance pads and Guitar Hero guitars, though there are still many people who find those to be gimmicks too.
Frankly, it's a gimmick if it doesn't fundamentally change the gameplay. Dance pads and guitar peripherals fundamentally changed rhythym games, making them much more like the real deal. The Wiimote worked for golf and tennis because it was a lot more like swinging a club or racket than pressing buttons on a controller is. But many games made a gimmick out of it: you'd use it as a regular pointing device through most of the game (fine), only for it to stop the game and get you to shake it like a tamborine or something. The usage has to fit naturally with the theme, otherwise there's no point to having it.
Motion recognition is great for a martial arts fighting game (try it in an arcade sometime), but how does it aid a racing game? Does it make more sense to select items from a menu by saying what they are rather than pointing at them with a controller? Are we going to end up using a regular controller to navigate the game world, only to be interrupted and told to run on the spot to beat some kind of minigame?
It's all about how it gets used by the developers. Some games will come out that use the technology well. Many others will come out that hack it on where it doesn't belong just so they can get into the market. It will probably take awhile before companies find a formula that works. Until then, we'll be inundated with gimmicky games. Whether Natal proves to be successful will depend on how long it takes for developers to find that sweet spot and how much crap we're flooded with before that happens.
Here's hoping it turns out.